Reinforcements or reinforcing structures for tires, and particularly for tires of vehicles of the heavy-goods type, are currently—and usually—made up of a stack of one or more plies conventionally known as “carcass plies”, “crown plies”, etc. This way of naming the reinforcements stems from the method of manufacture which consists in producing a series of semi-finished products in the form of plies, provided with threadlike reinforcing elements, often longitudinal, which are then assembled or stacked in order to build a tire preform. The plies are produced flat, with large dimensions, and then cut to suit the dimensions of a given product. The plies are also assembled, initially, substantially flat. The preform thus produced is then shaped into the toroidal profile typical of tires. The semi-finished products known as “finishing” products are then applied to the preform to obtain a product that is ready to be vulcanized.
A “conventional” type of method such as this entails, particularly during the phase of manufacturing the tire preform, the use of an anchoring element (generally a bead wire) which is used to anchor or secure the carcass reinforcement in the region of the beads of the tire. Thus, in this type of method, a portion of all the plies (or just some of the plies) that make up the carcass reinforcement is wrapped around a bead wire positioned in the bead of the tire. Thus, the carcass reinforcement is anchored in the bead.
The fact that this conventional type of method is widespread throughout the tire-manufacturing industry, in spite of there being numerous alternative ways of producing the plies and the assemblies, has led those skilled in the art to employ a vocabulary hinged on the method; hence the terminology generally accepted which in particular includes the terms “plies”, “carcass”, “bead wire”, “shaping” to denote the change from a flat profile to a toroidal profile, etc.
Nowadays, there are tires which do not, strictly speaking, have any “plies” or “bead wires” consistent with the above definitions. For example, document EP 0 582 196 describes tires manufactured without the use of semi-finished products in the form of plies. For example, the reinforcing elements of the various reinforcing structures are applied directly to the adjacent layers of rubber compounds, all of this being applied in successive layers to a toroidal core the shape of which allows a profile similar to the final profile of the tire being manufactured to be obtained directly. Thus, in this case, there are no longer any “semi-finished” products, or any “plies” or any “bead wires”. The base products, such as the rubber compounds and the reinforcing elements in the form of threads or filaments, are applied directly to the core. Because this core is of toroidal shape, there is no longer any need to shape the preform in order to change from a flat profile to a profile in the shape of a torus.
Furthermore, the tires described in that document do not have any “traditional” wrapping of the carcass ply around a bead wire. That type of anchorage is replaced by an arrangement whereby circumferential threads are positioned adjacent to the said sidewall reinforcing structure, everything being embedded in an anchoring or bonding rubber compound.
There are also methods of assembly onto a toroidal core that employ semi-finished products specifically adapted for rapid, effective and simple placement on a central core. Finally, it is also possible to use a hybrid comprising both certain semi-finished products for achieving certain architectural aspects (such as plies, bead wires, etc.) while others are achieved by applying compounds and/or reinforcing elements directly.
In this document, in order to take account of recent technological evolutions both in the field of manufacture and in the design of products, the conventional terms such as “plies” “bead wires”, etc., are advantageously replaced by terms which are neutral or independent of the type of method used. Thus, the term “carcass-type reinforcement” or “sidewall reinforcement” can be used to denote the reinforcing elements of a carcass ply in the conventional method and the corresponding reinforcing elements, generally applied to the sidewalls, of a tire produced according to a method that does not involve semi-finished products. The term “anchoring region”, for its part, can denote the “traditional” wrapping of the carcass ply around a bead wire in a conventional method, just as easily as it can denote the assembly formed by the circumferential reinforcing elements, the rubber compound and the adjacent sidewall reinforcing portions of a bottom region produced using a method that involves application onto a toroidal core.
Generally, in tires of the heavy-goods type, the carcass reinforcement is anchored on each side in the bead region and is radially surmounted by a crown reinforcement consisting of at least two layers, which are superposed and formed of threads or cords that are parallel within each layer. It may also comprise a layer of metal threads or cords with low extensibility making an angle of between 45° and 90° with the circumferential direction, this ply, known as the triangulation ply, being situated radially between the carcass reinforcement and the first crown ply known as the working ply, formed of parallel threads or cords at angles of at most 45° in terms of absolute value. The triangulation ply forms, with at least the said working ply, a triangulated reinforcement which, under the various stresses to which it is subjected, suffers little by way of deformation, the triangulation ply having the essential role of reacting the transverse compressive loads to which the collection of reinforcing elements is subjected in the region of the crown of the tire.
The crown reinforcement comprises at least one working layer; when the said crown reinforcement comprises at least two working layers, these are formed of inextensible metal reinforcing elements that are parallel to one another within each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. The said working layers that form the working reinforcement may even be covered with at least one layer known as a protective layer and formed of reinforcing elements that are advantageously made of metal and extensible, known as elastic elements.
In the case of tires for “heavy-goods” vehicles, just one protective layer is usually present and its protective elements are, in most cases, directed in the same direction and at the same angle in terms of absolute value as those of the reinforcing elements of the radially outermost and therefore radially adjacent working layer. In the case of construction machinery tires intended to run over fairly uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements in the radially inner protective layer being crossed with the inextensible reinforcing elements in the radially outer working layer adjacent to the said radially inner protective layer.
Such tires also usually comprise, in the beads, one or more layers of reinforcing elements known as stiffeners. These layers usually consist of reinforcing elements which, with respect to the circumferential direction, are oriented at an angle of smaller than 45°, and most often of smaller than 25°. These layers of reinforcing elements notably have the function of limiting the longitudinal movements of the materials of which the bead is made with respect to the rim of the wheel in order to limit premature wearing of the said bead.
Moreover, when the crown reinforcement is anchored around a bead wire, a process which consists in winding the carcass reinforcement at least partially around a bead wire in each of the beads, forming a turn-back extending up the sidewall to a greater or lesser extent, the layers of reinforcing elements or stiffener may also limit the onset of delamination between the turn-back of the carcass reinforcement and the materials based on polymer compounds surrounding the said turn-back. The problem is that this type of anchorage between the carcass reinforcement and the bead wire can, depending on the conditions of use, lead to risks of delamination between the turn-back of the carcass reinforcement and the materials based on polymer compounds surrounding the said turn-back.
These layers of reinforcing elements or stiffener are usually arranged axially on the outside of the turn-back of the carcass reinforcement and extend up the sidewall by a height that is greater than the height of the turn-back, notably in order to cover the free ends of the reinforcing elements in the said turn-back.
Cords are said to be inextensible when the said cords exhibit a relative elongation of at most 0.2% under a tensile force equal to 10% of the breaking strength.
Cords are said to be elastic when the said cords exhibit a relative elongation of at least 4% under a tensile force equal to the breaking strength.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is the direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it rotates under normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential mid-plane or equatorial plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
Certain current tires known as “road” tires are intended to run at high speed for increasingly long distances because of improvements to the road network and because of the growth of the motorway network throughout the world. All of the conditions under which a tire such as this is called upon to run undoubtedly allows the number of kilometers covered to be increased, as tire wear is lower, but on the other hand the endurance of this tire is thereby penalized.
It has actually become apparent that certain running conditions lead notably in both parts of the tire surrounding the beads, to degradation of the masses of polymer compounds, entailing tire replacement.